The shape of things to come(Photo: Jeff Blackler, Rex Features)

The 54.5 U.S. mpg fuel economy standard is just around the corner. Will we all have to drive cars like this?

David Booth

David Booth — The Fast Lane

One of the more optimistic assumptions of the recent drive for ever-increasing fuel economy — with the goal of reaching the Obama-mandated 54.5 (U.S.) miles per gallon fleet average by 2025 — is that traditional solutions like reducing engine size and lowering weight will be the major sacrifices that will have to be made to the gods of clean tailpipes. Unfortunately, increasing fuel economy that much will require more substantial changes, one that may alter our vehicles more dramatically than we think.

For instance, light weight alone will not generate anywhere near massive improvements in fuel economy envisaged. As an example of how difficult improvements will be, despite losing an incredible 250 kilograms thanks to its revolutionary (the first ever for a production SUV) aluminum frame, the 2013 Range Rover boasts but a nine per cent increase in fuel economy. It sounds even less impressive when you measure actual miles per gallon; after an engineering remake that just a few years ago would have been considered pioneering, the Range Rover's overall fuel economy increased from 14 mpg to ... 15 (U.S. figures used since Transport Canada has not yet rated this vehicle).

Land Rover aluminum structure(Photo: Land Rover)

Light is right for the all-aluminum Range Rover ... but it's not enough to make a difference all on its own

Making matters worse is that at least a part of those paltry gains has nothing to do with the weight loss, but the lower highway engine speeds attributable to the Range Rover's new eight-speed automatic transmission. It's impossible to know the transmission's exact contribution (mainly because, in announcing the overall fuel economy improvements of their new automobiles, automakers hate crediting less marketable technologies like multi-speed transmissions when they are trying to push sexier upgrades like turbocharged "Ecoboost" engines), but a little basic math suggests that the increased number of gears is, by itself, good for about a six percentage point boost in highway fuel economy. That leaves but two or three points to be credited to the weight reduction, an incredibly small figure that illustrates that simply building lighter vehicles will not, in and of itself, result in huge fuel economy improvements, particularly on the highway.

Nor will simply installing smaller engines in existing cars engender significant fuel efficiency gains. According to one Consumer Reports test, Ford's much ballyhooed substitution of a 3.5-litre, twin-turbocharged V6 for its traditionally V8 resulted in absolutely no reduction in fuel consumption. Jaguar, meanwhile, has even taken the measure to even greater lengths, introducing a 2.0-litre turbocharged four (the same as used in the Range Rover Evoque) is its full-sized XJ luxury sedan. Yet compared to the new 3.0-litre supercharged V6, the little four banger saves but 0.1 L/100 km in fuel consumption (9.4 L/100km for the 3.0 and 9.3 for the 2.0). Yes, in reducing performance from the 340 horsepower V6 to the 240-hp four, one saves about three ounces of fuel for every 100 kilometres driven. No wonder Jaguar has declined to import the 2.0L XJ to North America (mind you, the car is important in countries where taxation is based on engine size).

2013 Audi A6 2.0T(Photo: Audi)

Four-cylinder A6 posts good fuel economy numbers, but the difference is mainly in the city

In real-world evaluations, simply reducing the size of an engine in the same kind of full-sized car does not amount to any greater improvements. Audi's A6 is, for the first time in North America, available with the Volkswagen Group's ubiquitous 2.0-litre turbocharged four as an option to the long-running 3.0-litre supercharged V6.

The good news is that the big Audi suffers very little for the downsizing of engine. For most driving, they are all but indiscernible, even experienced autojournalists struggling to notice any performance disadvantage. Only when the throttle is matted does one notice the V6's superior performance. The bad news is that there's little to gain at the pumps. On the highway, the 2.0T reduces consumption from the 3.0T's 7.6 L/100 km to 7.3, about enough in 10,000 kilometres of highway cruising to save you enough for a nice bottle of Chablis. In the city, the improvement is slightly better, the average dropping from 13.9 L/100 km to 12.9; notable, but not planet-saving.

Why are the gains so paltry? One of the main reasons is that the most important fuel consumption criteria for highway fuel consumption is aerodynamics. At speed, how efficiently a car cuts through the air largely determines how much fuel it will consume. At a steady 100 km/h, weight is hardly a determining factor and while engine efficiency is important, the largest gains are to be made with slippery shapes.